1. Field of the Invention
This invention relates, broadly speaking, to a novel conductive adhesive sealant for bipolar fuel cell separator plate assemblies.
More particularly, this invention relates to a novel composition for a conductive adhesive sealant for bipolar fuel cell separator plate assemblies, based principally upon vinyl ester resins of a particular composition or range of compositions.
2. Description of the Prior Art
PBM (Polymer Electrolyte Membrane) fuel cells convert the reaction between hydrogen and oxygen gasses into electricity. Conductive separator plates are used to support the electrolyte membrane. Grooves in the plates distribute the gasses evenly to the electrodes deposited on the membrane. Electrons, generated at the anode, flow through the conductive plates to the adjacent cathode, where they combine with oxygen to form water using the protons from the adjacent cell. Electric current flows through a stack of contiguous plates and PBM membranes, and metallic collector plates at each end of the stack are used to collect the power generated. It is important to minimize resistive loss due to contact resistance in order to achieve optimum efficiency.
As licensee of Los Alamos National Laboratory under U.S. Pat. No. 6,248,467 (2001), Bulk Molding Compounds, Inc., the assignee of the present invention, supplies a graphite powder-filled, vinyl ester resin-based thermosetting molding compound, BMC 940, which can be compression molded to provide the net shape required for a separator plate geometry, in connection with which no machining is required.
Heat is generated in PEM fuel cell operation, and heat management is one of the aspects of stack engineering.
Heat typically is removed from the stack by circulating a liquid coolant through grooves between contiguous plates. Because the plate material is an excellent conductor of heat as well as of electricity, heat generated in the reaction is thus transferred to the liquid coolant and removed.
Two approaches to forming these coolant channels have been attempted, and problems have been associated with each approach.
In the first approach, an anode plate and a cathode plate are compressed together such that coolant channels are formed between them. In order to prevent leakage, elastomeric gasket material is used between the plates. There are two problems with this first approach. Firstly, the seal is subject to failure due to uneven compression or chemical attack by coolants. Secondly, electrons must bridge the contact points between the anode and the cathode, and since they are merely touching each other in this design, typically there is a relatively high degree of contact resistance which wastes stack power and generates additional unnecessary heat.
The second approach attempts to overcome these problems by permanently adhering the anode plate and the cathode plate using a liquid adhesive. Conductive liquid adhesives currently available typically use two-part epoxy binders filled with conductive material such as graphite. These systems must be used within a short period of time, minutes to hours. In order to achieve an effective seal, after application of the sealant to the plates has been accomplished, these systems require excessively long cycle times, up to several hours in duration. Finally, the bonded assembly has a through-plate resistance significantly higher than it would be had it been made from a single, solid piece of material.
As a matter of interest, U.S. Pat. No. 6,248,467 (2001) is directed to composite bipolar plates for electrochemical cells, consisting of a molded mixture of a vinyl ester resin and graphite powders. There is no disclosure in this patent of a conductive adhesive sealant for bipolar fuel cell separator plate assemblies.
As a matter of further interest, U.S. patent application Publication No. US2002/0055030 discloses a separator for a solid polymer type fuel cell produced by molding a vinyl ester series resin and graphite powder. There is no disclosure in this patent publication of a conductive adhesive sealant for bipolar fuel cell separator plate assemblies.
As yet another matter of further interest, U.S. Patent Application Publication No. US2002/0086198 discloses an electrically conductive resin composition comprising a vinyl ester resin, a radical polymerization initiator and carbonaceous filler, suitable for use as a separator for fuel cells. There is no disclosure in this patent publication of a conductive adhesive sealant for bipolar fuel cell separator plate assemblies.